The third season of the justifiably popular British import, created and written by Julian Fellowes, comes to PBS on Sunday with the first of seven new episodes set in 1920.

It is the dawn of a new age, not only for the residents of Downton Abbey, upstairs as well as downstairs, but for England as well. The Great War is over, and society is changing. Women are getting their hair bobbed and wearing their dresses shorter — well, the younger ones anyway: Certainly not the Dowager Countess (Maggie Smith).

The war has taken an economic toll on the nation and that includes the Crawley family. For generations, the Crawleys have depended on their tenant farmers for income, and the present Lord Grantham (Hugh Bonneville) was able to realize a cash infusion by marrying Cora (Elizabeth McGovern), an American heiress.

But at every turn, the old ways are being forced to give way to the new. Eldest daughter Mary (Michelle Dockery) is getting ready to marry third cousin once removed Matthew Crawley (Dan Stevens), but her father is worrying that the family may have to sell the abbey itself unless some financial solution can be found.

Perhaps that solution might be found by hitting up Cora’s visiting mother, Martha Levinson (Shirley MacLaine), a cross between Dolly Gallagher Levi and Molly Brown, who swans into the Abbey with her rough-hewn ways and a forthrightness that adds an extra inch to the Dowager’s frequently raised eyebrow.

Just as Lord Grantham battles to keep things from changing above stairs, butler Carson (Jim Carter) struggles to maintain decorum among the household staff. Footman Thomas (Rob James-Collier) is still scheming against fellow staff members, while the formidable O’Brien (Siobhan Finneran) is out to undercut Thomas in order to advance her nephew in the household pecking order.

Lady’s maid Anna Bates (Joanne Froggatt) vows never to give up until she proves that her husband, Bates (Brendan Coyle), was wrongly imprisoned for the murder of his first wife and continues to see the good in everyone at the Abbey, above and below stairs.

The third season will see changes at every level, with the death of a major character, the adjustments Mary and Matthew have to make to married life under the Abbey roof, changes in the management of the abbey and its properties, and the arrival of Lady Sybil (Jessica Brown Findlay) and her Irish husband, Tom (Allen Leech), the former chauffeur.

All of this makes for one delight-filled episode after another … very much in spite of weaknesses in the script.
In the past, it’s been easy to overlook other issues, such as the similarities not only between “Downton” and the Robert Altman film “Gosford Park,” whose script was also crafted by Fellowes, but also between “Downton” and “Upstairs Downstairs” as well. It would be less easy to overlook the flaws in the scripts for season 3 of “Downton,” were it not for the fact that we already know and love the major characters so well.

But if we allow ourselves some distance, Fellowes gets lazy, particularly in the early episodes of the third season, by occasionally advancing the plot at the expense of characterization. One major character gets a letter containing information that will have a far-reaching impact on his life, but doesn’t choose to open it until it’s convenient to do so in the company of another key figure in the story.

Really? If you got a letter telling you how your life was going to proceed for the foreseeable future, and change your destiny, would you wait to open it?

At another point, a character offers to help with the Crawleys’ financial problems yet, despite ample opportunity to clarify, fails to say, “Oh, by the way, that wouldn’t include actual money, you understand.”

Other characters demonstrate questionable inconsistency. Are a particular husband and wife truly devoted to each other and part of an indestructible partnership, or is the wife becoming a bit of a shrew? How does Tom’s character manage to adjust so easily to live within the upstairs formality of Downton life while still professing to be such an Irish rebel? Is the Dowager a tolerant modernist or a rock-ribbed traditionalist?

On the one hand, all of this smacks of the kind of convenience and coincidence often found in 19th century novels. Yet, in Dickens, for one, when a character needed to announce his or her intentions before following through with them, or even when one of Hardy’s doomed title characters was subject to fatalism, at least it was within range of our understanding of who the character is. That isn’t always the case in the third season of “Downton,” whose characters occasionally behave, well, uncharacteristically.

On the one hand, our love of the characters makes it more than possible to overlook the sloppiness of the scripts. On the other, though, it’s because we do know these characters so well that we notice the inconsistencies in the first place.
Again, none of this detracts significantly from our enjoyment of the series. But since it’s just been renewed for a fourth season, perhaps Fellowes can take more time with future scripts to let events play out more realistically and treat his characters with the respect they’ve earned by virtue of how well they were crafted in the first place.
David Wiegand is The San Francisco Chronicle’s executive features editor and TV critic.On TV
“Downton Abbey: Masterpiece: Classic.”
What: Season 3 premiere
When: 9 p.m. Sunday
Where: WMHT Ch. 17
★★★